The parents of Katherine Russell, the wife of late Boston Marathon bombing suspect Tamerlan Tsarnaev, were questioned for more than three hours before a federal grand jury in Boston on Thursday as part of a continuing Justice Department investigation into the April 15 attack, one of Russell's lawyers told NBC News.

Joshua Dratel, a lawyer for Russell, confirmed that his client's parents, Judith and Warren Russell, were separately questioned by federal prosecutors for a total of "three to four hours." The grand jury appearance at the U.S. District Courthouse in Boston was first reported by AP.

Dratel declined to discuss the nature of the questioning. But he told NBC News he believes prosecutors are still examining contacts his client had with her late husband, including in the days after the bombing. He said he has concerns she may still be indicted because of political pressure to do so -- although he does not believe the evidence would justify such a move.

"We don't see a basis for any charges based on the facts and the law," he said. But, he added, "I feel vulnerable based on who is going to be making the decision. We believe there may be people trying to manipulate the process." He did not identify who he believes that might be, but suggested there might be pressure being brought on the FBI by "members of Congress" and others.

Asked for comment on the Russells’ grand jury appearance, an FBI spokesman said only that the investigation into the attack is "ongoing."

Julia Malakie / AP file

Tamerlan Tsarnaev in a Feb. 17, 2010, file photo.

The U.S. Attorney's Office in Boston, which is overseeing the investigation, did not respond to requests for comment.

Russell, 24, and a convert to Islam, lived with her husband in the Cambridge, Mass., apartment where investigators believe Tsarnaev and his brother, Dzhokhar Tsarnaev, plotted the bombing that killed three people and wounded more than 250. Tamerlan Tsarnaev was killed in a shoot-out with police after the attack and his brother has since been indicted.

Russell's lawyers have repeatedly denied she had any knowledge of her husband's plans to bomb the marathon. Dratel said that he recently cut off cooperation with the FBI in the investigation because of concerns that agents were engaged in overly aggressive questioning of his client.